The term "heterogeneous plastic material", whenever used in the present specification and in the claims, means a mixture of thermoplastic and thermosetting polymers such as (high, medium, low density) polyethylene, polypropylene, polystyrene, foamed polystyrene, PVC, PET, ABS, polyester resins, polyamides, etc., obtained, after a first separation, from town solid wastes, or from a differentiated collection or deriving from the conversion industry as production wastes.
Reasons of ecological and environmental nature have led to take into due consideration the serious problem of the disposal of the plastic materials which are present in the town solid wastes.
In Italy, at present, the production of town solid wastes is growing by about 1% per year, and now it is evaluated at 18/20 million tons per year, 1.4 million tons of which (about 7.5%) are plastics.
The recycle of said high amount of material would allow to recover the residual use value thereof. That would result in economical advantages and, above all, in a decrease in the total waste volume and in the fresh material consumption. Furthermore, the possibility would be maintained to recover the respective energy content when, at the end of the life cycle, said material will be incinerated in a furnace according to a technology suitable for heat recovery.
A very important solution, from the applicability point of view, consists in the reuse of each component of the heterogeneous mixtures in combination with fresh products or in the selection of homogeneous fractions for the production of shaped bodies having physical/mechanical properties much higher than those of identical shaped bodies obtained from unselected mixtures.
The main problem associated with the recycling of the recovery plastic material is that of separating it into homogeneous fractions, preferably for the same type of polymer or, at least, for polymers thermally consistent with one another, by means of continuous and as much as possible automated processes.
At present, the mixes of recovered plastic material are substantially composed of polyolefins such as polyethylene, polypropylene, etc., which are almost always present in amounts greater than about 20% by weight, generally from 40% to 50%, of a mix substantially consisting of polyvinyl chloride (PVC), polystyrene, polyethylene terephthalate (PET), which in the aggregate represents about 50-60%, and of about 0-10% by weight of other products such as thermosetting resins, ABS, rubbers, acrylic resins, polyamides, etc.
The separation of these mixes by usual water flotation processes has provided not to be useful, because materials essentially consisting of polyolefins compatible with one another, float on the surface, while high specific weight materials, which comprise two reciprocally incompatible polymers such as PET and PVC, precipitate to the bottom.
PVC and PET are incompatible with each other as they are characterized by different processing temperatures in the molten state. In fact, PET melts at about 265.degree. C. while PVC, as is stated in "The Condensed Chemical Dictionary (Tenth Edition)" Gessner G. Hawlay, Van Nostrand Reinhold Company, 1981, decomposes at 148.degree. C. and generates toxic gases.